Chris---

There have been several excellent responses pointing out the different
humidity needs of different kinds of artifacts.  Often the artifact itself
may have conflicting needs (archaeological or ethnological artifacts being
classic examples), so the best bet might be to seek focused parameters from
institutions holding objects similar to yours.

I would add that several studies (see either Museum Forum or Barbara
Appelbaum's book on environmental control in museums, or any of several
others) suggest that variations in humidity are of greater concern than mean
levels.    So the mean may be less important than the deviation (sheesh, I
sound like a statistics prof!).  There's also a temporal aspect to consider.
Rapid fluctuations are more damaging than longer-term changes.   So
heretical as it might seem, you might be best advised to determine what
stable humidity can you maintain, rather than shooting for levels that are
theoretically ideal but in practice result in regular fluctuations to try
and attain a theoretical ideal.  Obviously you should consult a conservator
regarding both the practical "ideal" and what your best institutional
strategy might be.

Finally, changes in humidity are generally viewed as more damaging than
changes in temperature.  I mention this because rapid-cycle fluctuations in
humidity are often associated with HVAC activity (where intermittent HVAC
activity may keep temperatures stable but result in regular humidity
fluctuations).  Conversely, sometimes AC can ameliorate humidity blooms
associated with peak visitation or other short-term contributors to humidity
spikes.

Bottom line--determine an intersection between the RH you can reliably
maintain and that best suits the objects in your care, weighted toward those
most sensistive to humidity variations.  Go for stability, and if you have
to choose between stable RH and stable temp, opt for stable RH.

Hope it helps.

AB


Alex W. Barker, Curator of Archaeology
Dallas Museum of Natural History
PO Box 150349 Dallas, TX 75315-0349
(214) 421-3466 ext. 244 fax 214-428-4356
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